Positive displacement pumps, such as piston pumps and diaphragm pumps, typically displace an essentially constant volume of liquid with each stroke of the positive displacement pumping member. Pumps of this type are very useful for many applications, and a typical positive displacement pump is shown by way of example in Hartley U.S. Pat. No. 4,153,391.
For some applications and under some operating conditions, the characteristic of positive displacement pumps which causes them to displace a constant volume of liquid per stroke is undesirable. For example, when a positive displacement pump is used to supply a liquid for which there is a low demand relative to the output of the pump, the pump must operate under substantial back pressure unless some means is provided to correct this condition. A high back pressure tends to provide a heavy load on the motor which drives the pump.
One way to attempt to correct this is to cycle the pump on and off in response to demand as is commonly done in water supply systems for recreational vehicles. However, this technique is noisy and requires maximum current draw each time the pump is run. In addition, on-off cycling causes some variation in flow rate as the pressure changes between the pressure limits required to cycle the pump. This is particularly undesirable when the pump is used in beverage dispensing systems.
An effective way of varying the output from a positive displacement pump is to drive the pump with a wobble plate drive and to vary the angle of the wobble plate. One such construction is shown in Schoenmeyr U.S. Pat. No. 4,507,058, and the contents of this patent are incorporated by reference herein. In the construction shown in this patent, a coupler is rocked about a drive shaft against the biasing action of a resilient member to achieve variation in the wobble plate angle. Although this technique is desirable, for some applications, it is difficult to obtain sufficient force from the resilient member to transmit the desired torque.